Facile synthesis of 3D plum candy-like ZnCo₂O₄ microspheres as a high-performance anode for lithium ion batteries

Abstract

In this paper, 3D plum candy-like ZnCo₂O₄ microspheres (3D plum candy-like ZCO MSs) with nanoscale building blocks were synthesized by an ultrasonic spray pyrolysis technology and evaluated as anode materials for high-performance lithium ion batteries (LIBs). The uniform ZnCo₂O₄ microspheres exhibit plum candy-like architectures and are built from a large amount of interconnected nanoparticles with a diameter of approximately 38 nm. Owing to the unique hierarchical porous structure, the 3D plum candy-like ZCO MSs exhibit many advantageous properties such as their ability to facilitate the transport of Li⁺ and electrolytes by shortening the diffusion ways, to accommodate the mechanical stress and volume change associated with the Li⁺ insertion/extraction processes, and to improve the contact area between electrode and electrolyte, which are beneficial to improve the electrochemical performance. As a consequence, the ZnCo₂O₄ nanomaterials exhibit excellent cycling performance with a discharge capacity of 1030 mA h g⁻¹ after 110 cycles at 200 mA g⁻¹ and superior rate capability (769 mA h g⁻¹ at 2000 mA g⁻¹). In virtue of the simple synthesis method and excellent electrochemical performance, 3D porous ZCO MSs have huge potential as anode materials for the next-generation LIBs.

---

Introduction

Developing advanced electrode materials for Li-ion batteries (LIBs) to satisfy the increasing demand for portable power sources with high energy densities and power densities has attracted great attention.^1–5 Nowadays, transition metal oxides (TMOs) are emerging as promising electrode materials for LIBs owing to their high theoretical capacity (∼1000 mA h g⁻¹), which are typically three times those of conventional graphite/carbon-based electrode materials.^6–12 Among the transition metal oxides, Co₃O₄ has been widely investigated owing to its high theoretical capacity of ∼890 mA h g⁻¹. However, due to the fact that cobalt is toxic and expensive, numerous efforts have been made toward replacing the cobalt in Co₃O₄ by more cheaper and environmentally-friendly metal ions, which will not influence the good electrochemical performance. As a result, several ternary metal oxides have been studied as attractive electrode material for LIBs, such as NiCo₂O₄,¹³ MnCo₂O₄,¹⁴ FeCo₂O₄ (ref. 15) and ZnCo₂O₄.¹⁶ In particular, ZnCo₂O₄ has been a promising candidate electrode material as it could store lithium through not only a conversion reaction between Zn/Co species and Li but also the alloying reaction between Zn and Li, leading to a high theoretical capacity of ∼900 mA h g⁻¹.

However, similar to other transition metal oxides, ZnCo₂O₄ material also suffers from poor rate capability and inferior cycling stability, which is caused by its intrinsic low electronic/ionic conductivity and the drastic volume change during the discharge/charge process. It has been well established that nanosized anode materials provide enhanced electrochemical performance as they could offer advantages of not only increasing the contact area between the electrode and the electrolyte, but also shorting the Li⁺ and electrolyte diffusion pathways.^17–20 To date, various ZnCo₂O₄ electrode materials with different nanostructures and morphologies have been reported to improve the electrochemical performance, such as nanoparticles,²¹ nanowires,²² nanosheets,²³ nanorods²⁴ and microspheres.^25,26 Although some achievements have been made for ZnCo₂O₄ as anode for LIBs, the rate capability and cycling stability at high rate are still not satisfactory to meet the higher demands of electric vehicles with high energy densities. It has also been widely acknowledged that the structural and morphological features of electrode materials would influence the electrochemical performance on a large scale. Generally speaking, electrode materials with 3D hierarchical porous structure could not only sustain the large volume variation during the discharge/charge process for improving the cycling stability, but also possess a conductive network to transfer electrons effectively for the higher rate capability. Therefore, it is highly expected to prepare ZnCo₂O₄ electrode materials involving both hierarchical porous structure and nanometer-sized building blocks to further improve the electrochemical performance.

Recently, a simple spray pyrolysis process was successfully applied in the preparation of various materials including transition metal oxides owing to its simple operation, short production period and large-scale production.^8,27,28 For example, Zhang and co-workers²⁹ successfully developed 3D porous γ-Fe₂O₃@C nanocomposite using spray pyrolysis method and applied them in Na-ion batteries. Hong and co-workers³⁰ had synthesized SnO₂ yolk–shell-structured materials by the spray pyrolysis method and they showed an excellent electrochemical performance in LIBs. Therefore, in this study, we introduce the spray pyrolysis process to prepare 3D plum candy-like ZCO MSs for an advanced electrode material for LIBs. The as prepared products exhibit shrivelled microspherical shape and are composed by nanoparticles, which possess many structural merits. On the one hand, the interconnected nanowalls on the surface of products allow for the easy diffusion of Li⁺ and electrolyte by shortening the transport path ways. On the other hand, the hierarchical porous structure may not only guarantee the fine contact between electrolyte and active materials, but also accommodate the large volume variation associated with repeated lithium ion insertion/extraction processes. Meanwhile, the robust framework of secondary structure could improve the mechanical strength of electrode materials, which ensures the structural stability of anode materials during the long cycling process. Therefore, benefited from their intriguing structure characteristics, the 3D plum candy-like ZCO MSs exhibit an excellent cycling stability at a current density of 200 mA g⁻¹ and a superior high rate capability up to 2000 mA g⁻¹, indicating that the 3D plum candy-like ZCO MSs could serve as a promising anode material for high-performance LIBs.

Experimental

Sample preparation

The 3D plum candy-like ZCO MSs were prepared by an ultrasonic spray pyrolysis technique combined with thermal treatment at air. In brief, 0.1 M Zn(CH₃COO)₂ and 0.2 M Co(CH₃COO)₂ were dissolved in 50 mL deionized water. This solution was then sprayed to generate a large quantity of droplets with a 1.7 MHz ultrasonic spray generator. Subsequently, the droplets were carried through the tubular furnace at 500 °C using argon as carries gas with a flow rate of 2 L min⁻¹. Thermal decomposition of metal salt precursor yield Zn_xCo_1−xO microspheres. To obtain the finally ZnCo₂O₄ products with pure crystal structure, the as-prepared Zn_xCo_1−xO microspheres were continuously calcined to form black powders from green powders at 600 °C for 2 hours at air (Fig. S1†). The final ZnCo₂O₄ products were collected on the wall of quartz tube and washed by deionized water for several times followed with further drying in a vacuum oven at 60 °C overnight.

Materials characterization

Phase structures and morphologies of the samples were characterized by powder X-ray diffraction (XRD, Bruker D8 Advance Cu-Kα, λ = 1.5418 Å), a field-emission scanning electron microscope (FESEM, Hitachi S-4800) and a transmission electron microscopy (TEM, JEM-2100F) with an energy dispersive spectrometer (EDS) attachment. The surface area analysis was measured using the Brunauer–Emmett–Teller (BET) method with a Micromeritics 3 Flex surface characterization analyzer at 77 K. The chemical states of the samples were performed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha 1063) equipped with an Al Ka X-ray source. The X-ray photoelectron spectra of all the elements were calibrated using C 1s peak (284.6 eV) as a reference.

Electrochemical measurements

Electrochemical performance of 3D plum candy-like ZCO MSs was tested using 2025-type coin cells. The working electrode was prepared using active material, carbon black (Super-P-Li), and polymer binder (polyvinylidene fluoride; PVDF) with a weight ratio of 70 : 20 : 10 in N-methyl pyrrolidinone (NMP). The resulting slurry was coated uniformly onto a piece of copper foil and then dried under vacuum at 80 °C for 12 h. The thickness of coating is 150 nm and the density of active material is about 0.9 mg cm⁻². Lithium foil was used as an anode and 1 M LiPF₆ in a mixture of ethylene carbonate and diethyl carbonate (1 : 1) as the electrolyte. The cyclic voltammograms (CVs) was tested in the range of 0.01–3 V at a scanning rate of 0.1 mV s⁻¹ on CHI660E electrochemical workstation.

Result and discussion

The formation mechanism for the 3D plum candy-like ZCO MSs by one-step spray pyrolysis is shown in Fig. 1. Firstly, large amounts of droplets containing Zn²⁺ and Co²⁺ were generated by atomizing the precursor solution. Then, the droplets were carried into the tube furnace by the argon and heated up along with the solvent evaporation and decomposition process. During this process, micro-sized particles involving metal salt precursors were produced and subsequently decomposed into a plenty of metal oxide nanoparticles and CO₂ to form the Zn_xCo_1−xO microspheres. At the same time, due to the release of CO₂ gas from the microspheres, a driven force was produced to contract the hollow shell, which leads to form the shriveled microstructure. Ultimately, ZCO MSs with pure crystal structure were achieved by annealing at 600 °C for 2 h in air.

Fig. 1 The formation mechanism of 3D plum candy-like ZCO MSs.

As an intermediate for 3D plum candy-like ZCO MSs, the products after the spray pyrolysis process (Zn_xCo_1−xO) were characterized by XRD technique. As shown in Fig. S2,† the XRD patterns have mixed crystal structures of hexagonal ZnO, CoO and Co₃O₄ phases, and no diffraction peaks belonging to ZnCo₂O₄ were detected. The calcination of Zn_xCo_1−xO at air leads to the formation of ZnCo₂O₄. In Fig. 2, all intense peaks in the XRD patterns can be well ascribed to ZnCo₂O₄ crystal structure (JCPDS no. 23-1390), and the sharpness of the diffraction peaks indicates that the ZnCo₂O₄ phase in the products is well crystallized. Fig. 3 shows the typical SEM and TEM images of the as-prepared ZnCo₂O₄ products. As can be seen in Fig. 3a–c, the morphology of ZnCo₂O₄ products is shriveled microspherical in shape with uniform size of 1–1.5 μm, which exhibits plum candy-like structures. The 3D plum candy-like ZCO MSs are composed of numerous nano-sized particles and large amounts of pores can be obviously observed between them. The average diameter of ZnCo₂O₄ nanoparticles is 38 nm (Fig. S4†). Notably, interconnected nanowalls with the average size of 80 nm caused by the contraction of microspheres could be observed on the surface of products, which may accelerate the transport of lithium ions and electrons by shortening the diffusion ways. In addition, morphologies of as-prepared Zn_xCo_1−xO were also analyzed by SEM (Fig. S3†), which are similar to that of 3D plum candy-like ZCO MSs, indicating the thermally stability of products. The HRTEM image of ZnCo₂O₄ nanoparticles as shown in Fig. 3d reveals two sets of lattice fringes with interplanar spacings of 0.243, 0.286 nm corresponding to the (311) and (220) planes of spinel ZnCo₂O₄ phase, respectively. The elemental mapping of 3D plum candy-like ZCO MSs (Fig. 3e) performed using energy-dispersive X-ray spectroscopy (EDS) in scanning TEM mode shows the uniform distribution of Zn, Co and O elements in ZnCo₂O₄ microspheres.

Fig. 2 XRD diffraction patterns of as-prepared 3D plum candy-like ZCO MSs.

Fig. 3 (a and b) FESEM images at different magnifications; (c) TEM images; and (d) HRTEM images of 3D plum candy-like ZCO MSs; (e) the elemental mapping of Zn, Co and O of 3D plum candy-like ZCO MSs.

The porosity of 3D plum candy-like ZCO MSs was measured by the nitrogen (N₂) absorption–desorption isotherms at 77 K (Fig. 4), exhibiting a typical type IV isotherm with a surface area of 20.16 m² g⁻¹. The sample has a relatively broad pore size distribution ranging from 30–100 nm, suggesting the presence of mesoporous and macroporous structure. In addition, the pore volume of the products is determined to be 0.085 cm³ g⁻¹, which is mainly derived from the void space between nanoparticles as well as the surface sunken holes of the microspheres. Such a hierarchical porous structure could facilitate the diffusion of Li⁺ and electrolyte into the active materials and accommodate the large volume variation during the charge–discharge process, which results in the greatly promoted electrochemical performance.

Fig. 4 N₂ adsorption/desorption isotherm of 3D plum candy-like ZCO MSs.

The composition and the surface electronic state of 3D plum candy-like ZCO MSs were further investigated by the X-ray photoelectron spectroscopy (XPS) and the corresponding results were presented in Fig. 5. The survey spectrum shown in Fig. 5a indicates the presence of Zn, Co and O as well as C elements. The C 1s peak with the binging energy of 284.6 eV is used as the reference for calibration. The Zn 2p spectrum (Fig. 5b) exhibits two strong peaks with the binding energy of 1021.1 and 1044.1 eV, corresponding to Zn 2p_3/2 and Zn 2p_1/2, respectively. The binding energy difference between the two peaks is 23.0 eV, which is in agreement with the other reports.^31,32 In Fig. 5c, two major peaks located at 780.2 eV and 795.3 eV are associated with Co 2p_3/2 and Co 2p_1/2 peaks, respectively, with the binding energy difference of 15.1 eV.^33,34 This suggests that the valence of Co cation can be assigned a value of +3. It is worth noting that there are also two weak peaks with the binging energy of 790 eV and 805 eV, indicating the presence of Co²⁺.^35–37 The O 1s spectrum (Fig. 5d) could be divided into two peaks centered at 531.5 eV and 529.7 eV. The former peak can be attributed to the oxygen from hydroxide ions and the latter corresponds to the metal–oxygen bonds.^38,39

Fig. 5 XPS spectra for 3D plum candy-like ZCO MSs: (a) survey spectrum and high-resolution (b) Zn 2p, (c) Co 2p, (d) O 1s spectra.

The electrochemical performance of 3D plum candy-like ZCO MSs was investigated by cyclic voltammetry and galvanostatic charge–discharge cycling. Fig. 6a shows the first four consecutive cyclic voltammograms (CVs) of electrode materials at a scan rate of 0.1 mV s⁻¹ between 0.01 and 3.0 V. During the first cycle, two cathodic peaks at 1.42 V and 0.85 V are detected. The former peak (1.42 V) can be ascribed to lithium insertion into the crystal structure of porous ZnCo₂O₄ microspheres without structural change, and the sharp peak at 0.85 V is owing to the reduction of ZnCo₂O₄ to Zn and Co and the formation of a solid electrolyte interface (SEI) layer.^22,40,41 In the anodic process, two broad oxidation peaks are observed at 1.75 and 2.15 V, corresponding to the oxidation of Zn⁰ to Zn²⁺ and Co⁰ to Co³⁺, respectively. In the second cycle, the cathodic peak is gradually shifted from 0.85 V to 1.0 V and becomes much broader, while the anodic peak keeps almost the same as the first cycle without any changes. The overlapping of the CV curves in the subsequent cycles indicates the high reversibility of the electrochemical reactions. Based on the above analysis and previous reports,^22,23,42 the lithium insertion/extraction reactions for 3D plum candy-like ZCO MSs anodes are believed to be processed as follows:

ZnCo₂O₄ + 8Li⁺ + 8e⁻ → Zn + 2Co + 4Li₂O (1)

Zn + Li₂O ←→ ZnO + 2Li⁺ + 2e⁻ (2)

Co + Li₂O ←→ CoO + 2Li⁺ + 2e⁻ (3)

LiZn ←→ Zn + Li⁺ + e⁻ (4)

3CoO + Li₂O ←→ Co₃O₄ + 2Li⁺ + 2e⁻ (5)

Fig. 6 (a) The first four cyclic voltammogram (CV) curves of the 3D plum candy-like ZCO MSs at a scan rate of 0.1 mV s⁻¹; (b) discharge–charge curves of the electrode at a current density of 200 mA g⁻¹; (c) cycling performance at current densities of 200 and 500 mA g⁻¹; (d) the rate performance at various current densities; (e) discharge–charge curves of 3D plum candy-like ZCO MSs electrode at different current densities.

The electrochemical performance of 3D plum candy-like ZCO MSs had been further examined by galvanostatic discharge–charge tests at 200 mA g⁻¹ between 0.01 and 3 V, as shown in Fig. 6b. In the first discharge process, a well-defined long voltage plateau at approximately 1.0 V can be observed, which is analogous to previous reports.⁴³ The initial discharge and charge capacities are 1279 and 950 mA h g⁻¹, respectively, corresponding to a coulombic efficiency (CE) of 74.3%. The irreversible capacity losses of the electrode may be attributed to the formation of solid electrolyte interface (SEI) layer on the surface of electrode materials and the irreversible lithium insertion reaction.^44,45 After 50 and 100 cycles, the discharge capacities are 1037 and 1047 mA h g⁻¹, respectively, which are both higher than the theoretical capacity (903 mA h g⁻¹) of ZnCo₂O₄. This excess specific capacity might be due to the reactions such as the reversible growth of a polymer/gel-like film⁶ caused by decomposition of the electrolyte during the conversion reaction and the insertion of lithium ions into interfacial storage and acetylene back.^45,46

Fig. 6c shows the cycling performance of 3D plum candy-like ZCO MSs at current densities of 200 mA g⁻¹ and 500 mA g⁻¹. It is worth noting that the sample exhibits a distinct capacity increase before the initial 40 cycles at 200 mA g⁻¹, which normally derives from the activation process in the electrode.⁴⁷ After that, the discharge capacity is retained at 1030 mA h g⁻¹ after 110 cycles and the coulombic efficiency of the electrode is maintained around 98%. As expected, the 3D plum candy-like ZCO MSs also exhibits better cycling stability at a high current density of 500 mA g⁻¹. The discharge capacity of this sample maintains as high as 746 mA h g⁻¹ after 110 cycles, corresponding to capacity retention of 78.9% compared to the second discharge capacity. It should be noted that the hierarchical porous structure plays an important role in the excellent cycling performance. The porous nanostructures produced by the loosely-arranged nanoparticles could provide extra spaces for the Li-ions and electrons insertion/extraction during cycling process. In addition, the mesopores between the interconnected nanoparticles and the interior spaces of microspheres can provide a “buffer space” to alleviate the volume expansion during lithiation/delithiation process, which results in greatly promoted cycling performance. Fig. S5† shows the SEM images of 3D plum candy-like ZCO MSs electrode materials after discharging/charging for 50 cycles at a current density of 500 mA g⁻¹. It can be observed that slight pulverization exits in the electrode, which is cause by the drastic volume variation during the repeated discharge–charge processes. However, it is worth noting that the structure and morphology of most electrode materials can be well maintained, which further indicates the structural stability of 3D plum candy-like ZCO MSs electrode materials.

To further investigate the rate capability of 3D plum candy-like ZCO MSs, the electrode was evaluated at various current densities from 100 to 2000 mA g⁻¹, as shown in Fig. 6d. Benefited from the unique structure, a specific capacity of 1082 mA h g⁻¹ (Fig. 6e) is obtained at the current density of 100 mA g⁻¹. Even at a high current density of 2000 mA g⁻¹, the specific capacity could still be maintained as high as 769 mA h g⁻¹ (Fig. 6e) with a capacity retention of 71.7%, which is significantly better than the previously reported ZCO-based anodes (Table 1). When the current density turns back to 100 mA g⁻¹, an average capacity of 1026 mA h g⁻¹ could be recovered. This high rate capability indicates that the 3D plum candy-like ZCO MSs have a promising potential for the high rate anodes in LIBs.

Table 1 Comparison of the electrochemical performance of various ZnCo₂O₄ materials

Materials	Cycling stability (mA h g^−1)	Rate capability (mA h g^−1)	Ref.
ZnCo2O4 nanoparticles	801 at 100 mA g^−1 (100 cycles)	350 at 1000 mA g^−1	16
ZnCo2O4 nanowires	890 at 200 mA g^−1 (50 cycles)	347 at 800 mA g^−1	19
ZnCo2O4 nanorods	767.1 at 0.2 mA cm^−2 (50 cycles)		30
ZnCo2O4 microspheres	721 at 100 mA g^−1 (80 cycles)	435 at 2000 mA g^−1	21
ZnCo2O4 nanosheets	980 at 200 mA g^−1 (200 cycles)	372 at 2000 mA g^−1	18
ZnCo2O4/polypyrrole	615 at 100 mA g^−1 (100 cycles)	263 at 2000 mA g^−1	35
ZCO/CNF	656 at 0.5C (300 cycles)	225 mA h g^−1 at 2C	34
ZnO/ZnCo2O4/C	669 at 500 mA g^−1 (500 cycles)	715 at 1600 mA g^−1	28
3D plum candy-like ZCO MSs	1030 at 200 mA g^−1 (110 cycles)	769 at 2000 mA g^−1	This work
	746 at 500 mA g^−1 (110 cycles)

---

Considering that the cycling stability and rate capability highly depend upon the interfacial charge-transfer process and reaction kinetics in anode materials, the electrochemical impedance spectra (EIS) of the 3D plum candy-like ZCO MSs after different cycles was also measured. Fig. 7 presents the Nyquist plots obtained from the 3D plum candy-like ZCO MSs electrode, which consists of two partially semicircles in the high and medium frequency regions and an incline line in the low-frequency region. The incline line in the low-frequency corresponds to the Warburg diffusion process (W), representing the solid state diffusion of Li⁺ into the active materials, while the high-frequency semicircles can be attributed to both the contact resistance caused by the formation of SEI layer (R_sf) and the charge transfer resistance (R_ct).^48,49 Notably, the fresh cell exhibits the lowest values (24 Ω) of the combined contact resistance and the charge transfer resistance (R_sf+ct). And the R_sf+ct value increases to 36 Ω after the first cycle, further confirming the formation of SEI layer on the active materials after the first discharge–charge process. It is worth pointing out that no significant increase of the R_sf+ct value could be observed in subsequent cycles, indicating a relatively freely transport of lithium ions and electrons in the electrode/electrolyte interface. This result further demonstrates the stability of the 3D plum candy-like ZCO MSs electrode, which guarantees the stable cycling performance and the high capacity retention.

Fig. 7 EIS spectra of 3D plum candy-like ZCO MSs electrode after different cycles.

The outstanding electrochemical performance of 3D plum candy-like ZCO MSs benefits from several aspects of the unique hierarchical porous structure. Particularly, the uniform distribution of nanoparticles in the 3D plum candy-like ZCO MSs can provide more active sites for lithium storage, which may contribute to the improved lithium storage. Meanwhile, the interconnected nanowalls on the surface of microspheres could not only facilitate the transport of Li⁺ by decreasing the diffusion ways, but also fabricate a conductive network transfer the electrons effectively, leading to superior rate capability as well as high lithium storage. Furthermore, the abundant mesopores between the nanoparticles and surface sunken holes allow for the easy penetration of electrolyte into active materials, and provide buffer spaces to accommodate the large volume variation associated with the repeated Li⁺ insertion/extraction processes, devoting to the excellent cycling stability. Finally, the robust porous microspheres, as secondary superstructures, could significantly enhance the structure stability by alleviating the mechanical strength during the cycling process, which may also contribute greatly to the stable cycling performance.

Conclusions

In summary, we have investigated a facile and scalable method for the synthesis of 3D plum candy-like ZCO MSs using an ultrasonic spray pyrolysis method followed by calcination at air. This sample exhibit the morphology of shriveled microspheres, which are composed of nanometer-sized particles with an average size of 38 nm. This unique structure of 3D plum candy-like ZCO MSs is beneficial to facilitate the transport of Li⁺ and electron, accommodate the large volume variation, alleviate the mechanical strength, and improve the electrode–electrolyte contact area, which are favourable for enhancing the electrochemical performance for LIBs. As expected, the electrode shows a reversible capacity of 1030 mA h g⁻¹ after 110 cycles at 200 mA g⁻¹ and superior rate capability with a discharge capacity of 769 mA h g⁻¹ at 2000 mA g⁻¹. It might be served as promising anode materials for high performance LIBs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51272073, 51572078 and 51541203) and the Scientific Research Fund of Hunan Province (2015JJ2033).

References

1. J.-M. Tarascon and M. Armand, Nature, 2001, 144, 359–367.
2. Q. Xie, Y. Ma, X. Wang, D. Zeng, L. Wang, L. Mai and D. L. Peng, ACS Nano, 2016, 10, 1283–1291.
3. P. G. Bruce, B. Scrosati and J. M. Tarascon, Angew. Chem., 2008, 47, 2930–2946.
4. M. G. Kim and J. Cho, Adv. Funct. Mater., 2009, 19, 1497–1514.
5. Q. Xie, Y. Ma, D. Zeng, X. Zhang, L. Wang, G. Yue and D. L. Peng, ACS Appl. Mater. Interfaces, 2014, 6, 19895–19904.
6. S. H. Lee, S. H. Yu, J. E. Lee, A. Jin, D. J. Lee, N. Lee, H. Jo, K. Shin, T. Y. Ahn, Y. W. Kim, H. Choe, Y. E. Sung and T. Hyeon, Nano Lett., 2013, 13, 4249–4256.
7. D. Wang, Y. Yu, H. He, J. Wang, W. Zhou and H. D. Abruna, ACS Nano, 2015, 9, 1775–1781.
8. Q. Xie, Y. Ma, D. Zeng, L. Wang, G. Yue and D. L. Peng, Sci. Rep., 2015, 5, 8351.
9. H. Su, Y. F. Xu, S. C. Feng, Z. G. Wu, X. P. Sun, C. H. Shen, J. Q. Wang, J. T. Li, L. Huang and S. G. Sun, ACS Appl. Mater. Interfaces, 2015, 7, 8488–8494.
10. S. H. Choi and Y. C. Kang, ACS Appl. Mater. Interfaces, 2014, 6, 2312–2316.
11. Q. Xie, D. Zeng, Y. Ma, L. Lin, L. Wang and D.-L. Peng, Electrochim. Acta, 2015, 169, 283–290.
12. Q. Xie, X. Zhang, X. Wu, H. Wu, X. Liu, G. Yue, Y. Yang and D.-L. Peng, Electrochim. Acta, 2014, 125, 659–665.
13. L. Shen, L. Yu, X. Y. Yu, X. Zhang and X. W. Lou, Angew. Chem., 2015, 127, 1888–1892.
14. A. K. Mondal, D. Su, S. Chen, A. Ung, H. S. Kim and G. Wang, Chemistry, 2015, 21, 1526–1532.
15. L. Liu, Z. Hu, L. Sun, G. Gao and X. Liu, RSC Adv., 2015, 5, 36575–36581.
16. H. Chen, Q. Zhang, J. Wang, Q. Wang, X. Zhou, X. Li, Y. Yang and K. Zhang, Nano Energy, 2014, 10, 245–258.
17. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.-M. Tarascon, Nature, 2000, 407, 496–499.
18. Z. Wang, L. Zhou and X. W. D. Lou, Adv. Mater., 2012, 24, 1903–1911.
19. C. Yuan, H. B. Wu, Y. Xie and X. W. Lou, Angew. Chem., 2013, 52, 2–19.
20. A. L. M. Reddy, S. R. Gowda, M. M. Shaijumon and P. M. Ajayan, Adv. Mater., 2012, 24, 5045–5064.
21. W. Wang, Y. Yang, S. Yang, Z. Guo, C. Feng and X. Tang, Electrochim. Acta, 2015, 155, 297–304.
22. N. Du, Y. Xu, H. Zhang, J. Yu, C. Zhai and D. Yang, Inorg. Chem., 2011, 50, 3320–3324.
23. Y. Zhu, C. Cao, J. Zhang and X. Xu, J. Mater. Chem. A, 2015, 3, 9556–9564.
24. H. Liu and J. Wang, Electrochim. Acta, 2013, 92, 371–375.
25. C. Wu, J. Cai, Q. Zhang, X. Zhou, Y. Zhu, L. Li, P. Shen and K. Zhang, Electrochim. Acta, 2015, 169, 202–209.
26. L. Hu, B. Qu, C. Li, Y. Chen, L. Mei, D. Lei, L. Chen, Q. Li and T. Wang, J. Mater. Chem. A, 2013, 1, 5596.
27. Y. Xu, Q. Liu, Y. Zhu, Y. Liu, A. Langrock, M. R. Zachariah and C. Wang, Nano Lett., 2013, 13, 470–474.
28. S. H. Choi, J. H. Lee and Y. C. Kang, ACS Nano, 2015, 9, 10173–10185.
29. N. Zhang, X. Han, Y. Liu, X. Hu, Q. Zhao and J. Chen, Adv. Energy Mater., 2015, 5, 1401123.
30. Y. J. Hong, M. Y. Son and Y. C. Kang, Adv. Mater., 2013, 25, 2279–2283.
31. S. Hao, B. Zhang, S. Ball, M. Copley, Z. Xu, M. Srinivasan, K. Zhou, S. Mhaisalkar and Y. Huang, J. Power Sources, 2015, 294, 112–119.
32. X. Ge, Z. Li, C. Wang and L. Yin, ACS Appl. Mater. Interfaces, 2015, 7, 26633–26642.
33. H. Niu, X. Yang, H. Jiang, D. Zhou, X. Li, T. Zhang, J. Liu, Q. Wang and F. Qu, J. Mater. Chem. A, 2015, 3, 24082–24094.
34. L. Guo, Q. Ru, X. Song, S. Hu and Y. Mo, J. Mater. Chem. A, 2015, 3, 8683–8692.
35. D. Barreca and C. Massignan, Chem. Mater., 2001, 13, 588–593.
36. R. Dedryvère, S. Laruelle, S. Grugeon, P. Poizot, D. Gonbeau and J. M. Tarascon, Chem. Mater., 2004, 16, 1056–1061.
37. Q. Xie, F. Li, H. Guo, L. Wang, Y. Chen, G. Yue and D. L. Peng, ACS Appl. Mater. Interfaces, 2013, 5, 5508–5517.
38. C. Yuan, L. Zhang, L. Hou, L. Zhou, G. Pang and L. Lian, Chemistry, 2015, 21, 1262–1268.
39. R. Chen, Y. Hu, Z. Shen, Y. Chen, X. He, X. Zhang and Y. Zhang, ACS Appl. Mater. Interfaces, 2016, 8, 2591–2599.
40. J. Bai, X. Li, G. Liu, Y. Qian and S. Xiong, Adv. Funct. Mater., 2014, 24, 3012–3020.
41. Y. Sharma, N. Sharma, G. V. Subba Rao and B. V. R. Chowdari, Adv. Funct. Mater., 2007, 17, 2855–2861.
42. X. B. Zhong, H. Y. Wang, Z. Z. Yang, B. Jin and Q. C. Jiang, J. Power Sources, 2015, 296, 298–304.
43. L. Huang, G. H. Waller, Y. Ding, D. Chen, D. Ding, P. Xi, Z. L. Wang and M. Liu, Nano Energy, 2015, 11, 64–70.
44. W. Wei, S. Yang, H. Zhou, I. Lieberwirth, X. Feng and K. Mullen, Adv. Mater., 2013, 25, 2909–2914.
45. N. Wang, X. Ma, H. Xu, L. Chen, J. Yue, F. Niu, J. Yang and Y. Qian, Nano Energy, 2014, 6, 193–199.
46. Z. Bai, Y. Zhang, Y. Zhang, C. Guo and B. Tang, Electrochim. Acta, 2015, 159, 29–34.
47. G. Zhang, L. Yu, H. B. Wu, H. E. Hoster and X. W. Lou, Adv. Mater., 2012, 24, 4609–4613.
48. L. Li, Y. Cheah, Y. Ko, P. Teh, G. Wee, C. Wong, S. Peng and M. Srinivasan, J. Mater. Chem. A, 2013, 1, 10935–10941.
49. A. K. Mondal, D. Su, S. Chen, X. Xie and G. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 14827–14835.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17316k

---